The invention relates to a battery monitoring unit with a semiconductor power switch coupled into a battery lead.
A battery monitoring unit of the type mentioned above was introduced by the White Products company at the fifteenth meeting of "Elektronik im Kraftfahrzeug" (Electronics in the Motor Vehicle) in 1995 in Essen in the form of a modular unit for motor vehicle batteries, the modular unit being coupled to a battery terminal. The purpose of the semiconductor power switch there is to have the ability to switch off the battery current in a controlled manner, for example, as protection against unauthorized use of the vehicle or as short circuit protection.
German Patent document DE 43 39 568 A1 discloses an apparatus for determining the charge state of a battery. The apparatus carries out a charge balance of the battery by evaluating the charging and discharging currents and, with the help of a measurement, checks and corrects the off-load voltage of the battery. Furthermore, the use of this procedure for an on-board electrical supply system of the vehicle is shown, which comprises a generator, the output voltage of which is controlled by a voltage regulator, a battery, which is charged by the onboard generator, and electrical supply system consumers, which can be connected over switching circuits to the supply voltage made available by the generator or the battery. The method for determining the charge state of the battery is implemented in a computer, which receives suitable measured values, particularly with regard to the battery voltage, the off-load voltage, the charging current, the discharging current, as well as the temperature of the battery.
German Patent document 38 35 662 C2 discloses an MOS-FET used as a reverse battery protection circuit within an apparatus for energizing inductive consumers in a motor vehicle.
U.S. Pat. No. 5,084,633 describes a circuit arrangement for the bidirectional determination of current for a power MOS-FET, particularly for a DMOST. For this purpose, the respective DMOST is constructed so that it has a relatively low-ohm sampling resistance in series with its drain connection.
In the field of power FETs, which are composed of a plurality of integrated individual FET components connected in parallel, so-called sense FET circuit arrangements are known, which make it possible to measure the frequently comparatively high amperage flowing over the power FETs with relatively little loss. For this purpose, a few individual FET components are combined into a so-called sense FET part, while the large number of remaining individual FET components form the actual power part, which lies with its source-drain segment in the power circuit. By suitable wiring, it is ensured that a fraction of the current, which flows over the power segment and corresponds exactly to the ratio of the number of individual sense FET components to those of the individual power FET components, is diverted over the sense FET part. This diverted measurement current, which is consequently clearly less than that in the power segment, can be measured with little dissipation of power, for example, as a voltage drop, at a sense resistance. Such sense FET circuit arrangements are described, for example, in the German Patent document 195 20 735 A1 and in the literature cited there.
It is an object of the invention to solve the technical problem of making available a battery monitoring unit, which offers, with relatively little effort, the possibility of switching off the battery current in a controlled manner and, in addition, the possibility of determining the battery current bidirectionally, with which it is possible to realize the monitoring of the charge state of the battery.
Pursuant to the invention, this objective is accomplished by making available a battery monitoring unit with a semiconductor power switch, which is coupled in series with a battery lead. A sense FET circuit arrangement determines the battery current bidirectionally. The power FET part of the arrangement forms the semiconductor power switch.
This battery monitoring unit according to the invention contains a sense FET circuit arrangement, which is designed so that it is capable of determining the battery current, that is, the charging and discharging current of the battery bidirectionally with a sense FET part, so that, by monitoring the battery continuously, a balancing for the charging state of the battery becomes possible. At the same time, the power FET part of the sense FET circuit arrangement functions as a semiconductor power switch, which can be energized and is coupled with the battery lead and over which the battery current can be switched off as required, for example, as short-circuit protection or when the batteries of the vehicle are used as protection against unauthorized use of the vehicle.
In a further development of the invention, the battery monitoring unit is realized as a modular unit, which is positioned close to the battery terminal. Alternatively, it can be integrated with the associated battery terminal into a single component.
In the case of a preferred embodiment of the battery monitoring unit, the sense FET circuit arrangement comprises a suitable evaluation circuit, particularly in the form of a current mirror switching circuit, which transforms the current signal into an asymmetric, ground-related signal
In the case of another preferred embodiment of a battery monitoring unit, the sense FET circuit arrangement comprises a single sense FET group with two current decoupling paths as well as a current mirror circuit, connected to one of these paths, so that a bidirectional detection of current is attained by means of the single sense FET group.
In the case of a further developed battery monitoring unit, the sense FET circuit arrangement consists of two parallel, oppositely poled sense FET groups for achieving a bidirectional determination of current.
In the case of a still further developed battery monitoring unit, the possibility of an offset voltage adjustment is created for the sense FET group.
In the case of yet another preferred embodiment of a battery monitoring unit, the power FET part of the sense FET circuit arrangement, which is purposefully appropriately designed, serves additionally as a reverse battery protection circuit for which purpose the power FET part has two oppositely poled power FET groups, which are connected in series.
In the case of a further developed battery monitoring unit, the power FET part is divided into several groups which can be switched off separately. This permits the ratio between the load current and the sense current to be adapted to the magnitude of the load current.
Especially preferred embodiments of the invention as well as a conventional, basic sense FET circuit, which is intended to help in understanding the embodiments, are shown in the drawings and described below.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.